Weird Speedo Fault

HI I'M LOOKING FOR A BIT OF HELP I HAVE A FOCUS 2001 1.8 PETROL ZETEC.
1. FIRST FAULT THAT OCCURED WAS THE SPEEDOMETER WOULD GO RIGHT UP TO 140MPH THEN GO UP AND DOWN WITH THE BEAT OF MUSIC PLAYING IE;SURGING UP/DOWN.
2. SPEEDOMETER DOES NOT GO UP NOW, DRIVING ALONG THE CAR WILL LOSE MAYBE 15% POWER,RADIO SOUND DROP AND SPEEDO DROP TO ZERO
3. IF DIP CLUTCH TO CHANGE GEAR THE BATTERY LIGHT WILL COME ON BUT ONLY WHEN DIP CLUTCH.

just sounds like a lack of electrical power i.e speedo surging , batery light coming on when revs drop, music power being affected all points to alternator failure....get a multi meter on it whilst running and then put all lights and heated window on see if the voltage drops...should stay 13.5v to 14v